The present invention relates to a magnetron plasma processing apparatus for processing a substrate, such as a semiconductor wafer, with magnetron plasma.
Recently, a magnetron plasma etching apparatus has come into practical use, because it can produce plasma of a high density at a relatively low atmosphere and performs etching of a fine process. In this apparatus, a permanent magnet is arranged above the chamber, and a magnetic field which leaks from the permanent magnet is applied horizontally to a semiconductor wafer. Further, an RF (Radio Frequency) electric field is applied to perpendicularly cross the magnetic field. Thus, the magnetic field and electric field are utilized in co-operation so as to cause drift motion of electrons, by which etching is carried out at extremely high efficiency.
In such magnetron plasma, it is a magnetic field perpendicular to the electric field, i.e., horizontal and parallel to the semiconductor wafer, that contributes to the drift motion of electrons. However, in the above-described apparatus, a uniform and horizontal magnetic field is not always formed, and therefore a sufficient uniformity of plasma cannot be obtained. As a result, problems including the non-uniformity of the etching rate and the charge-up damage occur.
In order to avoid such problems, there is a demand of forming a uniform magnetic field parallel to the semiconductor wafer within the process space of the chamber. A dipole ring magnet is known as a magnet capable of generating such a magnetic field. As shown in FIG. 17, a dipole ring magnet 102 has a structure in which a plurality of anisotropic pole-shaped magnet segments 103 are disposed to be a ring-like format around a chamber 101. As the directions of magnetization of these magnet segments 103 are shifted gradually, a uniform horizontal magnetic field B is generated as a whole. FIG. 17 is a view (plan view) of the apparatus taken from above, and the proximal end side of the magnetic field direction is represented by N, the distal end side by S, and the positions arranged at angle of 90.degree. from these sides by E and W, respectively. Further, in FIG. 17, a semiconductor wafer is represented by reference numeral 100.
The horizontal magnetic field generated by such a dipole ring magnet is a field directing in only one direction from N to S in FIG. 17. Therefore, only with the magnetic field itself, electrons travel in a drift motion in one direction, causing a non-uniformity of the plasma density. More specifically, electrons travel in the direction of the outer product of the electric field and magnetic field, that is, they move from E to W in a drift motion, in the case where the electric field is generated to direct from above to below. As a result, a non-uniformity in the plasma density is caused such that it is low at E side and high at W side.
In order to avoid this, the dipole ring magnet is rotated along its angular direction so as to vary the direction of the drift motion of the electrons. However, with merely such an operation, the plasma density cannot be unified in a wide range.
Under these circumstances, a technique has been proposed, in which a gradient of the magnetic field is created in the direction from E towards W in FIG. 17, and the problem of the non-uniformity of plasma, caused by the drift motion of electrons, is solved (Jpn. Pat. KOKAI Publication No. 9-27278). In this technique, as shown in FIG. 18, the number of magnet segments on the W side is reduced, and thus a gradient of the magnetic field intensity is created in the direction from E towards W.
When the size of the semiconductor wafer is increased to 300 mm in diameter, the electron density on the W side becomes excessively low with the above-described design of the magnet, and therefore the plasma density of that section becomes low, thereby causing an non-uniformity of the plasma. However, if the electron density on the W side is increased to a sufficient level so as to avoid the above problem, the magnetic field at the E side must be further intensified. In this case, the uniformity of plasma is disturbed in the vicinity of the E side of the semiconductor wafer. Consequently, when an insulating film, such as an oxide film, is etched, a charge-up damage or the like may be caused.
Further, recently, there has been a demand of further downsizing the devices, and accordingly, there is a demand of a plasma etching process which is carried out at a lower pressure than usual. In order to perform an efficient plasma etching process at a low pressure, it is necessary to further increase the plasma density. In this case, although the magnetic field intensity is generally increased, but, as described before, a problem of charge-up damage may occur when an insulating film, such as oxide film, is etched. In order to avoid the charge-up damage, the intensity of the electric field should be limited up to about 200 gausses at the position where the wafer is present. Thus, when the gradient of the magnetic field is created with the above-described technique, the uniformity of the plasma can be improved; however at the same time, in order to avoid the charge-up damage, the plasma density cannot be fully increased, and therefore the etching rate becomes insufficient.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 7-197255 discloses another technique for solving the problem of the non-uniformity of plasma, by adjusting the magnetic field intensity in the dipole ring magnet shown in FIG. 17. In this publication, a slit 104 is made at the center in the longitudinal direction of each of a plurality of anisotropic columnar magnet segments 103 and the size of each gap is set to correspond to the position of the respective magnet segment in the dipole ring magnet. In this case, each magnet segment consists of a pair of upper and lower portions.
Jpn. Pat. Appln. KOKAI Publication No. 9-27277 discloses a technique of solving the problem of non-uniformity of the plasma density, which is caused by the drift of electrons described above. More specifically, in this reference, a slit is also made at the center in the longitudinal direction of each of a plurality of anisotropic columnar magnet segments and the size of each gap is set to correspond to the position of the respective magnet segment, as in Jpn. Pat. Appln. KOKAI Publication No. 7-197255. The size of the gap is set such that a gradient of the magnetic field intensity is created in the electron-drift direction, that is, the direction taken from E towards W in FIG. 17, and thus the problem of the non-uniformity of plasma, caused by the drift motion of electrons, can be solved.
However, with the techniques disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 7-197255 and 9-27277, the z-component (component perpendicular to the surface of the wafer) of the magnetic field is disturbed in the vicinity of the magnet segment in a plasma forming space, due to the gap made at the center of the anisotropic columnar magnetic segment. As a result, it is considered that the electron-drift direction may be reversed in the disturbed section, causing an adverse effect to the result of the process.